PROJECT SUMMARY In this competitive renewal we focus on characterizing a novel missense variant that was identified by genome- wide association analysis (GWAS) of obesity-related traits in Samoans during the previous funding period. This missense variant is highly associated with body mass index (BMI) with an effect size greater than any other known common obesity risk variant. The gene harboring this missense variant encodes a putative transcription factor (TF) that has recently been implicated in energy metabolism in Drosophila but remains poorly characterized, particularly in higher organisms. Preliminary data presented in this application indicate that overexpression of both the wild-type human gene and its missense variant in 3T3L1 adipocytes enhances adipogenesis, promotes lipid storage, and improves cell survival. In addition, overexpression of the missense variant promotes even greater lipid storage and reduces energy substrate oxidation, suggesting that it is a ?thrifty? variant. Given the enormous contribution of obesity to disease, additional studies are urgently needed to understand the mechanisms by which this missense variant contributes to obesity in humans. The Central Aim of this proposal is to determine how the TF gene and its missense variant contribute to energy homeostasis and to identify the transcriptional pathways mediating these effects. We will achieve this goal using integrated studies in cells, mice, and humans to understand the molecular, physiological, and clinical relevance of the missense variant. The following Specific Aims will be pursued: Aim 1 will identify and characterize the gene networks mediating the effects of the TF gene and its missense variant on energy homeostasis and energy substrate metabolism in cultured cells, a range of metabolically-relevant tissues from mice, and adipose tissue from 123 Samoans; Aim 2 will characterize the impact of the missense variant on whole body and tissue-specific energy homeostasis and energy substrate metabolism using variant-specific knockin mice; Aim 3 will more precisely characterize the impact of the missense variant on metabolic and behavioral traits that impact energy homeostasis in a subset of 500 Samoans GWAS participants who will be selected based on their genotype. These deeper phenotypes will be measured with new fieldwork in Samoa by re-contacting participants from the original GWAS study, during which metabolic and nutritional conditions/exposures analogous to those used in Aims 1 and 2 will be tested; Aim 4 will more fully characterize the missense variant using comprehensive statistical approaches including testing for selective signatures, testing for pleiotropy via multivariate analyses, performing pathway analyses, testing for missense variant x environment interactions with a focus on diet and physical activity, testing for TF gene x gene interactions focusing on genes identified in Aims 1 and 2, and testing for association with newly gathered phenotypes from Aim 3. Successful completion of these aims will promote the understanding of obesity and its downstream health outcomes as well as identify novel targets for pharmacological interventions.